Artificial bone graft implant

ABSTRACT

The present invention provides for an artificial bone graft implant for use as a replacement for living bone material in surgical procedures requiring the use of bone graft material. The implant has a body configured to be implanted into a prepared site in a patient&#39;s bone tissue, with the body having a pair of opposed outer surfaces defining the body. A first and a second porous portion form the body with the first and second porous portions having pores of different sizes such that the average pore size of the first porous portion is greater than the average pore size of the second porous portion. The first porous portion of the body is formed in the shape of a core, with the core being in contact with the opposed outer surfaces of the body, and the second porous portion of the body is formed in the shape of an outer shell. The pore size of the first porous portion of the implant allows for the ingrowth of bone tissue and the pore size of the second portion of the implant allows for a load bearing capacity similar to natural bone.

This is a continuation of application Ser. No. 08/473,658 filed Jun. 7,1995 and now abandoned.

FIELD OF THE INVENTION

The present invention relates to artificial bone graph implants and morespecifically to artificial bone graft implants constructed so as toallow bone ingrowth while maintaining a load-bearing strength similar tonatural bone.

BACKGROUND OF THE INVENTION

There are numerous surgical situations in which bone grafts are used aspart of the surgical procedure. For example, bone grafts are used infacial reconstruction, in repairing long bone defects, and in spinalsurgery such as intervertebral discectomy and fusion in which a bonegraft implant replaces a spinal disc in the intervertebral space.

Bone used for bone graft implants is often removed from another portionof a patient's body, which is called an autograft. A significantadvantage of using a patients own bone is the avoidance of tissuerejection, but harvesting bone also has its shortcomings. There is arisk to the patient in having a second surgical procedure (boneharvesting) performed at a secondary site which can lead to infection oradditional pain to the patient. Further, the bone harvesting site isweakened by the removal of the bone. Also, the bone graft implant maynot be perfectly shaped which can cause misplacement of the implant.This can lead to slippage or absorption of the implant, or failure ofthe implant to fuse with the bone it is in contact with.

Other options for a bone graft source is bone removed from cadavers,called allograft, or from an animal, called xenograft. While these kindsof bone grafts relieve the patient of having a secondary surgical siteas a possible source of infection or pain, this option carries a highincidence of graft rejection and an increased risk of the transmissionof communicable diseases.

An alternative to using living bone graft implants is the use of amanufactured implant made of a synthetic material that is biologicallycompatible with the body. With varying success, several syntheticcompositions and various geometries of such implants have been utilized.In some instances, the implanting surgery of such implants isaccomplished without difficulty, but the results can be unsatisfactorybecause any minor dents or imperfections in the implant can cause poorbone-to-implant bonding or an implant having a very high porosity cancollapse due to lack of mechanical strength. In other instances, theartificial implant requires a complex surgical procedure that isdifficult to perform and may not lead to correction of the problemagain, because of the above discussed side effects or dislocation of theartificial implant. Presently, no fully satisfactory artificial implantis known that can be implanted with a relatively straight-forwardprocedure.

Considerable study has been devoted to the development of materials thatcan be used for medical implants, including load-bearing implants, whileallowing ingrowth of new bone tissue into the implant. To be suitablefor this use, the material must meet several criteria, namelybiocompatibility, porosity which allows tissue ingrowth and a mechanicalstrength suitable to bearing loads expected of natural bone withoutgreatly exceed the natural bone's stiffness.

Several materials have been examined as potential implant materialsincluding ceramics, such as hydroxylapatite, Ca₁₀ (PO₄)₆ (OH)₂, hardenedpolymers and biocompatible metals. Hydroxylapatite (HAp) has been ofparticular interest because of its similarity to natural bone mineral,but it has only been used for low load bearing applications as pureporous HAp itself is relatively low in mechanical strength and may notserve as a good prosthetic material for high load bearing implants.

Studies have been directed at improving the mechanical strengthproperties of an HAp material in order to render it suitable as a highload bearing prosthetic material. European patent EP 0577342A1 toBonfield et al. discloses a sintered composite of HAp and abiocompatible glass based on CaO and P₂ O₅ that may be used in dentaland medical applications as a replacement for unmodified HAp. To date,improvements in the mechanical strength of HAp material has beenachieved at the expense of its porosity. Upon densification necessary toachieve adequate load bearing strength, the HAp material has a porositywhich is insufficient to provide the desired degree of bone ingrowth.

In a study entitled "Dense/porous Layered Apatite Ceramics Prepared byHIP Post Sintering," Materials Science, Vol. 8, No. 10, pp 1203(October, 1989), by Ioku et al., the preparation of layers of dense HApand porous HAp from two different types of HAp powder is discussed. Thisstructure is prepared by first densifying specially produced finecrystals of HAp with a post-sintering process employing hot isostaticpressing (HIP). Then a commercial, coarse HAp powder is molded in layerswith the densified HAp. Despite its being of academic interest, thistype of HAp structure is not suitable for fabrication into load bearingbone prosthetic device configurations in which natural bone ingrowth maybe achieved because of its lack of strength. However, Ioku suggests thatthe addition of zirconia whiskers into the dense HAp layer might providesome of the toughness necessary for hard-tissue prosthetic applications.

Still desired in the art is an artificial bone graft implant that isformed of a biocompatible mineral material similar to bone whichpossesses compressive strength close to that of natural bone whileproviding for ingrowth of bone tissue for permanent fixation.

SUMMARY OF THE INVENTION

The present invention provides an artificial bone graft implant formedof a biocompatible mineral material which possesses compressive strengthsimilar to that of natural bone and allows bone tissue ingrowth forpermanent fixations. The artificial bone graft implant is used as areplacement for living bone material in surgical procedures requiringthe use of bone graft material. The inventive implant has a bodyconfigured to be implanted into a prepared site in a patient's bonetissue, with the body having a pair of opposed outer surfaces definingthe body. A first and a second porous portion form the body with thefirst and second porous portions having pores of different sizes suchthat the average pore size of the first porous portion is greater thanthe average pore size of the second porous portion. The first porousportion of the body is formed in the shape of a core, with the corebeing in contact with the opposed outer surfaces of the body, and thesecond porous portion of the body is formed in the shape of an outershell. The pore size of the first porous portion of the implant allowsfor the ingrowth of bone tissue while the pore size of the secondportion of the implant allows for a load bearing capacity similar tonatural bone.

As will subsequently be described, the unique hybrid structure of adense outer shell and a porous core provides load bearing support whilesimultaneously allowing bone ingrowth. The implant of the invention maybe readily implanted by established surgical procedures, with minimalneed to alter known surgical procedures. The hybrid porous/denseconstruction of the implant ensures normal load bearing and supportthrough the eventual ingrowth of bone tissue, and minimizes thelikelihood of implant dislocation relative to adjacent bone tissueeither during surgery or during the post-operative fusion process.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be obtained when thedetailed description of exemplary embodiments set forth below isreviewed in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of the present invention in the form of aspinal disc implant;

FIG. 1A is a plan view of the posterior end of the implant of FIG. 1taken along lines 1A--1A;

FIG. 1B is a cross sectional view of the implant of FIG. 1 taken alonglines 1B--1B in FIG. 1A;

FIG. 1C is a cross sectional view of the implant of FIG. 1 taken alonglines 1C--1C in FIG. 1A;

FIG. 1D is a cross sectional view of the implant of FIG. 1A taken alonglines 1D--1D in FIG. 1A;

FIG. 2 is a perspective view of the present invention in the form of afemoral ring implant,

FIG. 2A is a plan view of the posterior end of the implant of FIG. 2taken along lines 2A--2A;

FIG. 2B is a cross sectional view of the implant of FIG. 2 taken alonglines 2B--2B in FIG. 2A;

FIG. 2C is a cross sectional view of the implant of FIG. 2 taken alonglines 2C--2C in FIG. 2;

FIG. 3 is a perspective view of the present invention in the form of analternate spinal disc implant;

FIG. 3A is a plan view of the posterior end of the implant of FIG. 3taken along lines 3A--3A in FIG. 3;

FIG. 3B is a cross sectional view of the implant of FIG. 3 taken alongthe lines of 3B--3B if FIG. 3A; and

FIG. 4 is a schematic top plan view of a human cervical vertebra with animplanted spinal disk implant of FIG. 1 indicated in solid lines.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The artificial bone graft implant of the present invention has two basicportions, each composed of a biocompatible microporous material. A firstportion is a core element and a second portion is a shell element thatpartially surrounds, and is bonded with, the core element through aninterface region. The core is formed of a highly porous compositionwhich allows for bone ingrowth and the shell is formed of a low porositydense composition which provides for mechanical strength. This resultsin the average pore size of the core being greater than the average poresize of the shell. Also, the percent porosity of the core will begreater that the percent porosity of the shell. The interface regionconnecting the two portions may have gradient pore sizes in which thegradient range goes from a large pore size adjacent the core element toa small pore size adjacent the shell element. The core element can beformed in any shape but in one preferred embodiment it is formed of twouniformly sized cores generally oblong in shape and spaced apart fromeach other. A second preferred embodiment has one core element generallyrectangular in shape.

The shell element can also have a porous coating on its outer surface topromote bone ingrowth over all or a portion of the shell element inaddition to the highly porous core element. Alternatively, the shellelement can be formed with a gradient of pore sizes rather than beingformed of a unitary low porosity dense composition. In this embodiment,a center portion of the shell element is of a dense or low porositywhich gradually changes to a high porosity outside surface. The coreelement remains the same in this alternative embodiment.

The inventive implant is made from a microporous material that, aftersurgical implantation, bonds to the natural bone of the patient to forma rigid structure. Such material encompasses, but is not limited to,biocompatible metalics, ceramics (including hydroxylapatite), polymers,and composite materials consisting of phosphate(s), bioactive glass(es),and bioresorbable polymer(s). The implant is preferably made from aceramic, most preferably a hydroxylapatite such as calciumhydroxylapatite, having a chemical formula Ca₁₀ (PO₄)₆ (OH)₂, availablefrom Smith & Nephew Richards, Inc, 1450 Brooks Road, Memphis, Tenn.38116 U.S.A. The use of such materials in implants is known in the art,see for example U.S. Pat. No. 4,863,476, whose disclosure isincorporated in its entirety by reference herein.

The dense portion of the preferred hydroxylapatite implant can be formedby pressing the dry HAp powder which is followed by sintering. Theamount of pressure required for the pressing is dictated by the shape ofthe implant, but is typically in the range of 1000 to 2000 psi (6.9 to14.8 MPa). The pressure is used to consolidate the powder and maximizepacking. The optimal sintering protocol is dependent on the size andshape of the green (unsintered) part. The sintering could, but does notnecessarily, include simultaneous use of heat and isostatic pressure(HIP). Sintering atmospheres can include argon, nitrogen, air andvacuum. The porous portion of the preferred hydroxylapatite implantinvolves the addition of bubbling agents, such as hydrogen peroxide, toa HAp slurry. The slurry is then dried and sintered.

The hybrid dense/high porosity structure of the inventive implant can beproduced by a two-step process wherein the dense and highly porousportions are produced separately (as described above) then combined withthe interface and sintered to produce the final hybrid implant.

Alternatively, the hybrid dense/highly porous implant can be produced ina one-step process by injection molding a HAp slimy containing a binderthat will burn out during sintering which will create the differentporosity of the core and shell.

In an alternative embodiment, the inventive implant can be formed of aunitary structure having a gradient of pore sizes. The preferredgradient consists of a dense or low porosity center which graduallychanges to a high porosity outside surface. The average pore size of thehighly porous region would range from 100 to 800 microns. Bone requiresa minimum pore size before ingrowth can occur. The maximum size would belimited by the strength requirements of the implant. The percentporosity at which the implant structure is considered to have highporosity is generally between about 30% porosity (or 70% dense) to 40%porosity (or 60% dense) which would still allow the implant to maintainthe required mechanical strength. However, the maximum amount of percentporosity would most likely need to be lower than 40% in order tomaintain adequate strength of the implant structure.

The artificial bone graft implant of the present invention can be formedin any desirable shape for use in surgical procedures requiring a bonegraft implant, such as facial reconstruction, the repair of long bonedefects, and spinal surgery. For example, in spinal surgery, bone graftimplants are frequently used when a fusion is done as part of anintervertebral discectomy procedure. During the fusion procedure, thebone graft implant is inserted into the intervertebral space after thedisc is removed.

An embodiment of the present invention as an artificial spinal discimplant 10 is illustrated in FIGS. 1, 1A-D. As shown, implant 10 has twoopposed lateral surfaces 12, 14 with an anterior end 16 that taperstoward a posterior end 18. The implant includes a pair of high porosityinner cores or inserts 20, 22 that extend between the opposed lateralsurfaces 12, 14, a low porosity dense shell 24 surrounding the cores 20,22, and a region of gradient porosity 26 can separate the cores 20, 22and shell 24. The cores 20, 22 are generally oblong in shape with curvedend portions and are generally spaced equal-distant apart from eachother and the outer surface of the opposed lateral surfaces 12, 14.

A second embodiment of the invention as a femoral ring implant 10A, isillustrated in FIGS. 2, 2A-2C. Implant 10A has one central porous core28 extending between the opposed lateral surfaces 12, 14 and a denseshell 30. The central core 28 can be separated from the dense shell 30by a region of gradient porosity 32. The central core 28 is generallyrectangular in shape but could also be generally oval in shape. Thefemoral ring implant 10A can be used to repair long bones or it can beused as a spinal disc replacement such as implant 10.

The inventive implant formed of a unitary structure having a gradient ofpore sizes is illustrated in FIGS. 3, 3A, 3B as implant 10B. Implant 10Bhas a dense or low porosity center 32 between the opposed lateralsurfaces 12, 14 which gradually changes to a high porosity outer portion34, as shown in FIG. 3B. The implant 10 is illustrated in FIG. 4 as itwould appear when implanted between two cervical spinal vertebrae V(with only one vertebrae being shown) as a replacement for a spinaldisc.

In the preferred embodiment of the artificial bone graft implant, theimplant is formed of HAp including the core, shell, and gradientregions. The implant may also be made of calcium phosphate or othermicroporous ceramics, polymers and composite materials. The dense HApelement has a mechanical strength sufficient to bear the loads normallyexperienced by natural bones, and a compressive strength similar to thenatural bones of the body. The porous HAp core element allows fornatural bone ingrowth which facilitates permanent fixation of theimplant after implantation in natural bone.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited except as by theappended claims.

What is claimed is:
 1. An implant for replacing the supporting functionof an intervertebral space defined between adjacent vertebrae,comprising a prosthetic implant member dimensioned for insertion withina space provided by at least partial removal of the intervertebral disc,the implant member including upper and lower surfaces for respectivelyengaging upper and lower vertebral portions and defining an axisextending between the upper and lower surfaces, the upper and lowersurfaces defining a height therebetween along the axis sufficient tospan the intervertebral space and maintain the upper and lower vertebralportions in predetermined spaced relation, the implant member defining atransverse dimension transverse to the axis, the transverse dimensiongreater than the height of the implant member, the implant memberincluding a first implant portion and a second implant portion, thefirst implant portion comprising a microporous material formed to definea predetermined configuration and having a first relatively high averageporosity to facilitate rapid bone ingrowth within the first implantportion, the first implant portion extending through the upper and lowersurfaces of the implant member, the second implant portion comprising amicroporous material formed to define a predetermined configuration andhaving a second relatively low average porosity such that the secondimplant portion is characterized by having sufficient strength tosupport and maintain the adjacent vertebrae in spaced relation duringpermanent fixation of the implant member within the adjacent vertebrae,the second porosity being less than about 40%. the implant portionfurther including a third implant portion disposed between the first andsecond implant portions, the third implant portion having a thirdaverage porosity ranging between the first porosity and the secondporosity.
 2. The implant of claim 1 wherein the second porosity of thesecond implant portion is less than about 40%.
 3. The implant of claim 1wherein the first implant portion is disposed in a central area of theimplant member and the second implant portion is disposed in aperipheral area of the implant member.
 4. The implant of claim 3including a pair of the first implant portions, each being disposed inthe central area of the implant member.
 5. The implant of claim 2wherein the second implant portion includes a porous coating to promoterapid bone ingrowth.
 6. The implant of claim 2 wherein the implantmember comprises a biocompatible microporous material selected from thegroup consisting of polymers, ceramics and composite materials.
 7. Theimplant of claim 6 wherein the microporous material is selected from thegroup consisting of calcium phosphate, bioactive glass, bioresorbablepolymers, and hydroxylapatite.